The present invention relates to railway track circuits.
Track circuits are a well-established means of train detection and can also be used to provide a level of broken-rail detection. A fundamental difficulty with track circuits on modem electrified railways is that they must share the railway track with the traction return, and track circuits have consistently evolved to provide better immunity to interference from traction systems. Another key concern for track circuit signals is cross-coupling between tracks, which could result in one track erroneously accepting a signal from another track. Over recent history (the last 20 years) various track circuits have evolved that use Frequency Shift Keying (FSK) to form a distinct electrical signal that is transmitted along the track. EP-A-0 165 048 discloses a coded track circuit system using FSK as a carrier mechanism. Early FSK track circuits used relatively simple generators and receivers. Further enhancements have been made to such receivers to improve the discrimination of the FSK signal and to such transmitters to generate a more unique FSK signal.
Existing FSK systems use various FSK modulation techniques to develop a signal with some level of uniqueness from any other track circuit and from the signals generated in the traction return system.
Various modulation techniques for railway track circuits are also disclosed in WO 01/11356, U.S. Pat. No. 4,582,279, U.S. Pat. No. 4,498,650, U.S. Pat. No. 4,065,081, U.S. Pat. No. 4,015,082, SU-A-1592204 and CA-A-1 149 918.